1. Field of the invention
The present invention relates to scanning devices, for example those used in image analysis, observation or watching systems, or else used for example in aerial reconnaissance. These scanning devices, comprising for example a mirror, scan a field of view about an axis of displacement and send the image of this field back to a detector device via focussing means.
Such scanning devices find an application in any optical deviation measurement, direction finding, range finding system or else any infrared, visible or ultraviolet metrology device.
More particularly, such a scanning device finds an application in line scanners.
A scanner usually comprises a mirror with several faces reflecting an incident light beam scanning the landscape, this mirror being controlled by such scanning devices, the reflected beam being then received by means of a strip or small mosaic of detectors.
To provide a high performance scanner, three essential requirements must be combined:
to have a wide angle scanning system, PA0 to increase the number of faces of the rotary mirror or prism, PA0 to reduce the diameter of the prism for a given dimension of the pupil.
In order to obtain the maximum number of lines scanned per unit of time and for each detection element, for a minimum power driving the reflecting prism, the scanning angle of the incident scanning beam as well as its speed must be increased.
2. Description of the Prior Art
Known scanning devices may for example be of the type shown in FIG. 1 or in FIG. 2
In the example of FIG. 1, the reflected or output beam rotates after reflection on the mirror. The axis of rotation of the mirror merges with axis A. The reflected beam then rotates at the same speed as the rotating mirror, the incident fixed ray being outside the plane in which the beam rotates.
In the example of FIG. 2, the mirror rotates about an axis perpendicular to the plane of the sheet and the reflected beam rotates twice as fast as the rotating mirror; the fixed ray R.sub.f is in the plane where the beam rotates.
These known systems each have advantages:
For system 1, it allows wide angles of rotation of the beam rotating about axis A.
For system 2, the maximum angle of deviation of the rotating beam is limited to (360.degree..times.2)/N where N is the number of faces of the multi-face mirror, i.e. twice that of system 1.
However, these known systems have great drawbacks which the present invention strives to overcome.
In the first case, the maximum angle of deviation is appreciably less than 360.degree./N.
In the second case, the fixed ray being in the plane of the rotating beam, the faces of the mirror must be large so as to avoid vignetting and keep a constant oblique pupil.